A nitrogen vacancy (small circles) within a diamond crystal shows promise as a "bit" for quantum computers in part because of its great sensitivity to magnetic fields—a sensitivity that also could enable MRI-like studies on objects as small as living cells or single molecules. When green light strikes the nitrogen vacancy, it fluoresces red; detecting variations in this fluorescence permit scientists to extract its information. Credit: J. Taylor, NIST

The team's work has the long-term goal of developing quantum computers,
but it has borne fruit that may have more immediate application in medical science.
Their finding that a candidate “quantum bit” has great sensitivity
to magnetic fields hints that MRI-like devices that can probe individual drug
molecules and living cells may be possible.

The candidate system, formed from a nitrogen atom lodged within a diamond crystal,
is promising not only because it can sense atomic-scale variations in magnetism,
but also because it functions at room temperature. Most other such devices used
either in quantum computation or for magnetic sensing must be cooled to nearly
absolute zero to operate, making it difficult to place them near live tissue.
However, using the nitrogen as a sensor or switch could sidestep that limitation.

Diamond, which is formed of pure carbon, occasionally has minute imperfections
within its crystalline lattice. A common impurity is a “nitrogen vacancy”,
in which two carbon atoms are replaced by a single atom of nitrogen, leaving
the other carbon atom's space vacant. Nitrogen vacancies are in part responsible
for diamond's famed luster, for they are actually fluorescent: when green
light strikes them, the nitrogen atom's two excitable unpaired electrons
glow a brilliant red.

The team can use slight variations in this fluorescence to determine the magnetic
spin of a single electron in the nitrogen. Spin is a quantum property that has
a value of either “up” or “down,” and therefore could
represent one or zero in binary computation. The team's recent achievement
was to transfer this quantum information repeatedly between the nitrogen electron
and the nuclei of adjacent carbon atoms, forming a small circuit capable of
logic operations. Reading a quantum bit's spin information—a fundamental
task for a quantum computer—has been a daunting challenge, but the team
demonstrated that by transferring the information back and forth between the
electron and the nuclei, the information could be amplified, making it much
easier to read.

Still, NIST theoretical physicist Jacob Taylor said the findings are “evolutionary,
not revolutionary” for the quantum computing field and that the medical
world may reap practical benefits from the discovery long before a working quantum
computer is built. He envisions diamond-tipped sensors performing magnetic resonance
tests on individual cells within the body, or on single molecules drug companies
want to investigate—a sort of MRI scanner for the microscopic. “That's
commonly thought not to be possible because in both of these cases the magnetic
fields are so small,” Taylor says. “But this technique has very
low toxicity and can be done at room temperature. It could potentially look
inside a single cell and allow us to visualize what's happening in different
spots.”

The Harvard University-based team also includes scientists from the Joint Quantum
Institute (a partnership of NIST and the University of Maryland), the Massachusetts
Institute of Technology and Texas A&M University.